e., BM raised to the power of 0.75 in adult mammals and 0.83 in suckling young (Brody 1945, Kleiber

1975, Oftedal 1984, Riek 2008). RCMR in adults and neonates can be expressed as: (4) The ratio between RCMRneo and RCMRad (1.74/0.428) is ca. 4, indicating that the relative demand of the brain is approximately four times higher in neonates compared with adult Weddell seals. The brain is only one of the tissues that require glucose as a metabolic substrate (Cahill and Owen 1968). Red blood cells (RBC) in aggregate represent a volume of ca. 3 L in a newborn Weddell seal pup (Burns and Castellini 1996). Assuming a glycolytic rate of about 1–2 mol glucose per hour per liter of RBC (Jacquez 1984), estimated RBC glucose consumption

in a Weddell seal pup is about 10–20 g glucose per day, or one-third to two-thirds that of the brain (28 g/d; Eq. (3)). Depending Tanespimycin on the efficiency of recycling of glucose taken up by RBC (Cahill 2006), the estimated daily glucose requirement of a suckling Weddell seal pup is about 30–50 g/d. This is a conservative estimate, because it does not include minor additional demands by other glucose-dependent tissues such AZD3965 in vitro as spinal cord, peripheral nerves, renal medulla, or bone marrow (Cahill and Owen 1968). The glucose requirements of suckling pups must ultimately be supplied by the mother via milk constituents. We have hypothesized that pup glucose requirements—mostly to supply the large brain—place an evolutionary premium on secretion of sugar in phocid milks, and found that there is good agreement between the estimated pup DGB and milk sugar consumed by pups (Eisert et al. 2013). 上海皓元 Weddell seal milk contains on average 1.1% sugar in early lactation (0–14 d postpartum), primarily as lactose-based oligosaccharides (Eisert et al. 2013). Assuming a milk yield of 3.54 kg/d (Tedman and Green 1987), Weddell seal mothers provide ca. 39 g sugar per day to nursing young during early lactation, consistent with an estimated daily glucose requirement of 30–50 g. Although oxidation

of milk fat and protein by pups could provide glycerol and amino acids as substrates for glucose synthesis (Hall et al. 1976, Jungas et al. 1992, Eisert 2011), partitioning of glycerol and amino acids into gluconeogenesis rather than tissue deposition would reduce growth efficiency, with adverse consequences for growth rate and weaning mass. Reduction in weaning mass can in turn affect juvenile survival (Proffitt et al. 2008). The provision of 39 g milk sugar per day is of comparable magnitude to the glucose demand of the adult brain (Eq. (2)), and places a substantial burden on the mother. Lactating Weddell seals fast during the first few weeks postpartum (Eisert et al. 2005), and thus all glucose that is metabolized or exported as milk sugar must be generated, via gluconeogenesis, by catabolism of maternal tissues (Oftedal 1993, Eisert et al. 2013).